Various air fuel ratio detecting devices have been proposed which make use of characteristics of the oxygen concentration cell action and the oxygen ion pumping action of zirconia as disclosed, for example, in Japanese patent laid-open No. 56-130649. Use of an air fuel ratio detecting device of the type disclosed allows detection of a wide range of air fuel ratios.
However, the accuracy of such a conventional air fuel ratio detecting device is not sufficiently high as to allow the air fuel ratio detecting device to be used for detecting the stoichiometric air fuel ratio, for example, for a ternary catalyzer, due to its structure.